1. Field of the Invention
The present invention relates to a wet-type image formation apparatus, in particular, a wet-type image formation apparatus that controls an image formation condition based on image density of a patch image.
2. Description of the Related Art
A wet-type image formation apparatus includes a photoconductor, a developing roller, and the like. An amount of toner supplied from the developing roller onto the photoconductor to develop an electrostatic latent image is dependent on an amount of toner on the developing roller (amount of toner conveyed to a developing portion). Each of characteristics of the photoconductor and the developing roller is likely to be varied by being affected by a change in environment around the apparatus, a period of use of the apparatus, or the like. The variation in each characteristic leads to variation in the amount of the toner supplied to the electrostatic latent image, thereby affecting the density of an image finally formed on a recording medium.
In order to attain a desired value for the density of an image formed on a recording medium irrespective of the variation in each characteristic of the photoconductor and the developing roller, the amount of the toner on the developing roller (amount of liquid developer) is controlled, for example. By controlling the amount of the toner on the developing roller (the amount of the toner conveyed to the developing portion), a developing characteristic is changed, whereby an image having a desired image density can be formed on a recording medium.
In order to form such an image having a desired image density on a recording medium, a predetermined developing characteristic is also required. In the predetermined developing characteristic, developing efficiency is saturated when a contrast potential (also referred to as “developing contrast potential”) is set at a certain value or more, whereby the amount of the toner supplied onto the photoconductor becomes constant. Image formation is performed under a condition that such a developing characteristic is obtained. In order to obtain such a developing characteristic, it is necessary to control the amount of the toner on the developing roller (the amount of the toner conveyed to the developing portion), for example.
It is also necessary to control an charge amount of the toner in the liquid developer conveyed to the developing portion in order to obtain the above-described developing characteristic. The charge amount of the toner has influences over not only the developing characteristic (developing efficiency) but also dot reproducibility and image uniformity, so that the charge amount of the toner is preferably higher. However, if the charge amount of the toner is higher than necessary, the contrast potential for saturating the developing efficiency becomes also higher, so that the charge amount of the toner needs to be set at an appropriate value.
As described above, in the wet-type image formation apparatus, in order to stably form a high-quality image, it is general to control both the amount of the liquid developer on the developing roller and the charge amount of the toner conveyed to the developing portion (see Japanese Laid-Open Patent Publication No. 2010-204467, Japanese Laid-Open Patent Publication No. 2008-170602, Japanese Laid-Open Patent Publication No. 2011-175052, Japanese Laid-Open Patent Publication No. 2012-063411, Japanese Laid-Open Patent Publication No. 2009-015351, Japanese Laid-Open Patent Publication No. 2004-117847, and Japanese Laid-Open Patent Publication No. 2004-117846). The control for them is performed immediately after starting the apparatus, or the like. The density of a patch image is detected, and then based on the detection result, the amount of the liquid developer on the developing roller and the charge amount of the toner conveyed to the developing portion are adjusted.
Generally, the amount of the liquid developer on the developing roller is adjusted, and thereafter the charge amount of the toner conveyed to the developing portion is adjusted. Specifically, first, by setting the charge amount of the toner at a low value and setting the contrast potential at a high value, the developing characteristic for saturating the developing efficiency is attained. The amount of the liquid developer on the developing roller is adjusted such that the image density of a patch image upon saturation falls within a target range. After adjusting the amount of the liquid developer on the developing roller, the charge amount of the toner is adjusted to a value near the upper limit with which the predetermined developing efficiency can be maintained with a standard contrast potential. In this way, the amount of the liquid developer on the developing roller and the charge amount of the toner are adjusted to appropriate values, thereby obtaining an ideal developing characteristic (standard condition) with which a desired image density can be obtained.